The invention relates to receiving acoustically telemetered signals and more particularly to receiving such signals in the presence of background noise.
Systems have previously been developed for acoustically telemetering signals in boreholes. Typical of these systems are those disclosed by Applicant in U.S. Pat. Nos. 3,889,228 and 3,930,220 and application Ser. No. 390,833.
Acoustical telemetry systems of which those used in boreholes are illustrative are often characterized by high background noise. In the borehole systems considerable noise is generated by drilling, including that generated by the bit, by rubbing of the drill pipe on the casing, by rotation of the drilling table and by various machines on the platform. The systems referred to above describe the use of repeaters to compensate for signal attenuation in the drill pipe. Each repeater, as well as the surface equipment, employs a receiver that must distinguish telemetered signals from the background noise, which can be as much as 10 times larger in amplitude than the signals.
The systems referred to above employ a receiver that incorporates phase-locked loops to supplement noise rejection of cascaded active filters. In many cases, phase-locked-loops are effective, but noise pulses which characteristically occur in long pipes, can cause damped oscillations in the loop which prevent it from locking on the coherent signal. Furthermore, the voltage controlled oscillators of commercial phase-locked-loop units are designed to "pull" with the input frequency over a range of about .+-.10 Hz. thereby providing no noise rejection in that range.
In a later application (Ser. No. 441,620), Applicant discloses a noise rejection system in which a pair of storage capacitors are switched into a circuit across the source of alternating signals during respective opposite half cycles of the signal wave. The voltage utilized for switching is obtained from the output of an oscillator which is synchronized by and with the received signal, if such signal is within the pass band of a narrow pass band filter. Frequencies outside the desired pass band are rejected by an active band pass filter with positive feedback to cause a 90.degree. phase shift for frequencies outside the pass band. This system is effective, but in some cases of high noise levels at which it is desired that the receiver operate, it may be subject to overloading.